1. Field
The present invention relates to a nonaqueous-electrolyte battery and a battery assembly employing the same.
2. Description of the Related Art
Nonaqueous-electrolyte batteries which are charged/discharged based on the movement of lithium ions between the negative electrode and positive electrode are being extensively investigated and developed as batteries having a high energy density. Nonaqueous-electrolyte batteries employing a lithium-transition metal composite oxide as a positive active material and a carbonaceous substance as a negative active material have already been commercialized. The lithium-transition metal composite oxide generally contains cobalt, manganese, nickel, or the like as the transition metal.
On the other hand, nonaqueous-electrolyte batteries employing a material having a higher lithium insertion/release potential than carbonaceous substances, such as, e.g., a lithium-titanium composite oxide (about 1.55 V vs. Li/Li+), as a negative active material are being investigated in recent years (see, for example, Japanese Patent No. 3866740 and JP-A 9-199179 (KOKAI)). Lithium-titanium composite oxides change little in volume with charge/discharge and hence have excellent cycle characteristics. Such nonaqueous-electrolyte batteries are theoretically free from lithium metal deposition and can hence be charged at a high current.
In the case where a carbonaceous substance is used as a negative active material, a protective coating film (SEI: solid electrolyte interface) is formed on the surface of the negative electrode. This protective coating film is formed by the reductional decomposition of the electrolyte during the first charge. Although the coating film itself functions as a resistive ingredient to impair life characteristics, it has the effect of inhibiting the negative active material from excessively reacting with the electrolyte to thereby inhibit self-discharge. Consequently, for realizing satisfactory battery characteristics, it is necessary to form a protective coating film which has low resistance and is highly effective in inhibiting self-discharge. In order to accomplish such subject, investigations are being made on modifications of the protective coating film by adding various substances to electrolytes.
On the other hand, negative electrodes employing a lithium-titanium composite oxide are less apt to undergo the formation of a protective coating film thereon as compared with negative electrodes employing a carbonaceous substance. This is because the lithium-titanium composite oxide has a high lithium insertion/release potential and is less apt to reduce the electrolyte. It has been found that although negative electrodes employing a lithium-titanium composite oxide are less apt to be coated with a coating film and hence have low resistance, they are susceptible to self-discharge.
There has hence been a desire for a technique for forming a satisfactory protective coating film on the surface of a negative electrode to inhibit self-discharge without changing resistance even when the negative electrode employs a material having a high lithium insertion/release potential.